A physicist, a biologist, and a mathematician are sitting in a café and notice people going into and coming out of the house across the street. First they see two people going into the house. Time passes. After a while, they notice three people coming out of the house.
The physicist says, “The initial measurement was not accurate.”
The biologist says, “They must have reproduced.”
The mathematician says, “If one person enters the house, then it will be empty again.”
The Teacher's Amusement Center 